The invention relates to the continuous casting of metal strips, in particular in aluminium or aluminium alloy. The invention particularly relates to a cooling circuit for rolls used in the continuous casting of metal strips, with which it is possible in particular to reduce thermal ovalisation (or wearing-out of round) occurring in said rolls during use.
As illustrated in the cross-section diagram in FIG. 1, a continuous casting machine for metal strips generally contains at least two identical rolls (1A and 1B) positioned face to face, separated by a space (or gap) E having the thickness of the metal strip to be produced, which rotate in reverse direction in relation to one another. The metal 2 is supplied in liquid state from one side of the space by means of a channel 6, while the strip 3 leaves by the other side at its nominal thickness Eo. The metal solidifies between the two rolls at a point which is known as the solidification line 5.
With this type of system, it is possible to produce strips having a thickness ranging from a few centimeters to a few millimeters or less.
FIG. 2 shows the general structure of a roll of the state of the art. FIG. 2a is a cross-section in the rolling zone 20 that is to say in that part of the roll which comprises the shell. FIG. 2b corresponds to a longitudinal section view along the line of section I-Ixe2x80x2 of FIG. 2a. 
A roll 1 typically comprises a cylindrical body 10 which, in its central part, is surrounded by a shell 11 intended to receive the molten metal and used to roll the strip, and cooling means. It is necessary for the rolls to be efficiently cooled during the rolling operation.
Cooling is usually conducted by means of a cooling liquid, typically water, circulating in at least one cooling circuit 12 placed inside the roll body 10. This circuit comprises at least a first conduit 13 intended to bring cold water F and at least a second conduit 14 intended to evacuate the heated water C. These conduits are essentially in the form of blind holes parallel to the axis 4 of the roll which emerge on one of its ends, the other end being sealed, and they extend the whole length of the shell 11. A plurality of radial pipes 15, 16 of smaller diameter connect each conduit 13, 14 to a corresponding collector 17, 18 which is in the form of a groove positioned just below the inner surface of the shell 11 and arranged parallel to the axis 4 of the roll. The collectors 17, 18 are connected to a plurality of ring-shaped channels 19 positioned just below the shell 11 in a plane that is transversal to the axis 4 of the roll. The ring-shaped channels and the collectors are generally machined-finished on the peripheral surface of the roll body 10.
Each cold-water supply conduit 13, 131, 132, and the corresponding radial pipes 15, 151, 152 and so-called distribution collector 17, 171, 172 form a cold water supply circuit. Also, each evacuation conduit for the heated water 14, 141, 142 and the corresponding radial pipes 16, 161, 162 and so-called evacuation collector 18, 191, 192 form a heated water evacuation circuit. FIG. 3 illustrates the alternation, in peripheral direction, of the supply and evacuation collectors for roll bodies of the prior art (only a few ring-shaped channels 19 are shown for reasons of clarity). Typically each radial pipe simultaneously supplies 5 separate ring channels.
The cooling water is injected into the circuit via the cold-water supply conduits 131, 132, . . . , distributes itself in distribution collectors 171, 172, 173 . . . , via a first series of radial pipes 151, 152, . . . , enters into thermal contact with the shell at the location of collectors 171, 172, . . . , and ring channels 19, thereby ensuring cooling of the shell, is then collected in the evacuation collectors 181, 182, 183 . . . , via a second series of radial pipes 161, 162, . . . , and is then evacuated via evacuation conduits 141, 142, . . . The arrows in FIGS. 2a and 2b indicate the direction of circulation of the cooling liquid.
Generally, the rolls comprise an identical number of cold-water supply circuits and heated water evacuation circuits. The number of pairs of supply and evacuation conduits is typically two, three or four. These conduits, and their corresponding channels, are arranged symmetrically in the roll body. The case illustrated in FIG. 2 comprises two pairs of circuits which are arranged in alternate manner and staggered by 90xc2x0. In the event of three or four pairs of circuits, the angle is respectively 60xc2x0 or 45xc2x0.
With the cooling circuits of the state of the art, cold and hot zones occur in the shell and roll in the vicinity of the collectors and channels of cold water supply and heated water evacuation. This heterogeneity in temperature, which may reach 4xc2x0 C., causes expansion which generates roll deformation called ovalisation or wearing-out of round. This leads to cyclical irregularities in the thickness of the cast metal strip thereby deteriorating quality. The problems caused by this defect increase with the decreasing thinness of the cast strip.
Temperature heterogeneity also modifies the real heat exchange coefficient between the metal and the shell, which produces a variation in thickness even if there is no roll deformation.
The applicant has therefore set out to find effective means, that are easy to produce or implement and are not costly, with which it is possible to overcome or minimize the temperature differences in the roll, in order to improve the quality and regularity of the thickness of the cast strip.
In order to solve this problem, the applicant put forward the solution, in French application FR 2 723 014 (corresponding to European patent application EP 694 356 and American patent U.S. Pat. No. 5,642,772), to periodically reverse the direction of circulation of the cooling liquid in the roll body, the cold liquid supply circuit becoming the evacuation circuit for the heated liquid, and vice versa. This solution, with which it is possible to considerably reduce wearing-out of round without having to change the rolls, nonetheless requires adaptation of the outer cooling circuit and the operating mode of the machine. In particular, the transitory operating conditions and/or the frequency of reversing the direction of circulation are dependent upon the type of alloy.
The applicant therefore set out to find solutions which remedy the drawbacks of the prior art and which in particular are able to reduce or even eliminate resulting temperature heterogeneity and variations in strip thickness.
The roll body of the invention for a continuous casting machine is able, in its central partxe2x80x94the so-called rolling zone, to carry a cylindrical shell and comprises a cooling circuit, which circuit comprises at least one cooling-liquid supply conduit, at least one cooling-liquid evacuation conduit, at least one distribution collector, at least one evacuation collector, at least one distribution pipe connecting each collector to the corresponding conduit, and a plurality of ring channels connecting the supply and evacuation collectors, said collectors and ring channels being used to place the cooling liquid circulating in said circuit in contact with the inner surface of the shell to ensure its cooling, and is characterized in that the collectors are arranged such as to produce, both in the peripheral direction and in the longitudinal direction, an alternation of distribution collectors and evacuation collectors.
The applicant had the idea of modifying the inner cooling circuit of the rolls so as to provide the alternation, preferably the close alternation, of the cold liquid arrival zones F and the heated liquid evacuation zones C, in both directions of the surface of the shell, that is to say both in the peripheral direction and in the longitudinal direction.
The applicant considers that this particular configuration of the cooling circuit, which does not significantly increase product manufacturing costs, produces an alternation of cold and hot zones below the inner surface of the shell which is able to promote a substantial reduction in temperature heterogeneity of the outer surface of the shell. The applicant also found that, surprisingly, the recourse to a plurality of collectors leads to greater uniformity in the flow of cooling liquid in the channels.
According to the preferred embodiment of the invention, the collectors are in the form of grooves, whose length is distinctly smaller than the length Lf of the shell, and which are aligned according to generatrix lines, angularly equidistant, and are connected to the supply and evacuation conduits in such manner as to produce an arrangement of the collectors in a regular network, even a chequered pattern.
A further object of the invention is a roll for a continuous casting machine comprising a shell and a roll body of the invention.
A further object of the invention is a continuous casting machine comprising at least one roll of the invention.
Yet a further object of the invention is a method for cooling continuous casting rolls in which the direction of circulation of the cooling liquid circulating in at least one roll of the invention is periodically reversed.